Integrated circuits generally obtain their operating clock during normal operation from a crystal oscillator. During standby operation, integrated circuits are often supplied with the operator clock by an oscillator circuit or a phase shifter oscillator circuit, since these circuits are distinguished by a low current consumption.
Oscillator circuits have a charge storage device which is alternately charged and discharged by an upward-integration current source and by a downward-integration current source, respectively. The charge storage device is furthermore connected to a comparator, which measures the charge state of the charge storage device and, upon reaching a predetermined upper comparator threshold, changes over from the charging process to the discharging process. Correspondingly, in the event of the charge state falling to a lower determined comparator threshold, the discharging process is deactivated and the charging process is activated instead.
Such oscillator circuits which are suitable for integration into an integrated circuit are described for example in the articles “A 1.2 μm CMOS Current-Controlled Oscillator” by Michael P. Flynn and Sverre U. Lidholm, published in IEEE Journal of Solid-State Circuits, volume 27, No. 7, July 1992, pages 982–987, and “A Novel CMOS Multivibrator” by I. M. Filanovsky and H. Baltes, published in Analog Integrated Circuits and Signal Processing, volume 2, 1992, pages 217–222, and “A novel low voltage low power oscillator as a capacitive sensor interface for portable applications” by Giuseppe Ferri and Pierpaolo De Laurentiis, published in Sensors and Actuators, volume 76, 1999, pages 437–441. In the oscillator circuits presented in these articles, a capacitor is in each case provided as the charge storage device.
The principle of a univibrator, on which the oscillator circuits are based, is illustrated in figure 6.50 on page 618 of the book “Halbleiter-Schaltungstechnik” [“Semiconductor circuitry”] by Ulrich Tietze and Christoph Schenk, published by Springer-Verlag, Berlin, 1999, 11th edition.
Applications of an oscillator circuit in the field of telecommunications require a high frequency stability. Therefore, the free-running frequency of an oscillator circuit, with the crystal frequency, is adjusted by digital divider ratios. Furthermore, the frequency generated by an oscillator circuit is intended to be largely independent of temperature fluctuations, operator voltage fluctuations, phase noise and also technology variations. Moreover, the oscillator circuit is intended to require a low operating voltage, have a low operating voltage dependence, take up little chip area and be able to be implemented in an integrated circuit.
The abovementioned quality requirements made of oscillator circuits also apply to applications in sensors. However, adjustment to a very precise crystal frequency is not usually necessary in applications of this type.
In order to obtain a greatest possible stability of the oscillator frequency with respect to temperature fluctuations, it is known to provide oscillator circuits with resistors having different temperature coefficients. However, what is disadvantageous about such compensation of temperature fluctuations is that technologically governed variations both of the temperature coefficients and of the absolute values of the resistors used bring about only a low temperature stability of the oscillator frequency.
Furthermore, it is known to bring about a temperature stability of the oscillator frequency by means of additional external components, such as e.g. external resistors, with low temperature coefficients or by means of adjustment on the wafer using an EEPROM or by means of zener zapping. What is common to these known measures is a large production outlay with the high costs associated therewith.